Virtual Vector Modulation and Capacitor Voltage Control for Coupled Eight-Switch Three-Phase Three-Level Inverter With Full Lagging Reactive Power Flow Capability

Due to the requirement of low cost in photovoltaic (PV) applications, an eight-switch three-phase three-level inverter (TP-TLI) and its modulation strategy have been proposed. However, it can only operate under high power factor (PF) conditions with this modulation strategy. In this article, the reactive power flow capability of each voltage vector is analyzed for the eight-switch TP-TLI. Based on this, a novel virtual space vector modulation (VSVM) is proposed. In each sector, it defines one virtual small vector and uses the vectors which are viable for full operation range of lagging PF. Hence, the proposed VSVM achieves full operation range of lagging PF. Besides, an active neutral point voltage control (NPVC) strategy is also proposed. Only one coefficient needs to be calculated, so the calculation is simple. In addition, it considers both the NPV fluctuation and the NP current introduced by the selected virtual small vector, and it can provide zero, positive, and negative NP current. Hence, it is accurate to control the NPV, and it could balance the NPV without obvious dc offset and ac ripples under all PF conditions and modulation indexes. The proposed modulation and control strategies are verified by experiment results with a 3 kVA prototype.